1. Field of the Invention
The invention relates generally to a mass flow measurement and control, and more particularly, to a mass flow measurement and control device based on the Coriolis force effect and having an integrated flow control valve with associated sense, control and communication electronics.
2. Description of Related Art
Mass flow measurement based on the Coriolis force effect is achieved in the following manner. The Coriolis force results in the effect of a mass moving in an established direction and then being forced to change direction with a vector component normal to the established direction of flow. This can be expressed by the following equation:
{overscore (F)}C={overscore (M)}xc3x97{overscore (xcfx89)}
Where {overscore (F)}C (the Coriolis force vector) is the result of the cross product of {overscore (M)} (the mass flow vector) and {overscore (xcfx89)} (the angular velocity vector).
In a rotating system, the angular velocity vector is aligned along the axis of rotation. Using the xe2x80x9cRight Hand Rulexe2x80x9d, the fingers define the direction of rotation and the thumb, extended, defines the angular velocity vector direction. In the case of the typical Coriolis force flow sensor, a tube, through which fluid flow is to be established, is vibrated. Often the tube is in the shape of one or more loops. The loop shape is such that the mass flow vector is directed in opposite directions at different parts of the loop. The tube loops may, for example, be xe2x80x9cUxe2x80x9d shaped, rectangular, triangular or xe2x80x9cdeltaxe2x80x9d shaped or coiled. In the special case of a straight tube, there are two simultaneous angular velocity vectors that are coincident to the anchor points of the tube while the mass flow vector is in a single direction.
The angular velocity vector changes directions since, in a vibrating system, the direction of rotation changes. The result is that, at any given time, the Coriolis force is acting in opposite directions where the mass flow vectors or the angular velocity vectors are directed in opposite directions. Since the angular velocity vector is constantly is changing due to the vibrating system, the Coriolis force is also constantly changing. The result is a dynamic twisting motion being imposed on top of the oscillating motion of the tube. The magnitude of twist is proportional to the mass flow for a given angular velocity.
Mass flow measurement is achieved by measuring the twist in the sensor tube due to the Coriolis force generated by a fluid moving through the sensor tube. Typical known devices use pick off sensors comprising magnet and coil pairs located on the flow tube where the Coriolis force""s induced displacement is expected to be greatest. The coil and magnet are mounted on opposing structures, for example, the magnet is mounted on the tube and the coil is mounted on the stationary package wall. The magnet will move in and out of the coil, inducing a current in the coil. This current is proportional to the velocity of the magnet relative to the coil. Since this is a velocity measurement, the velocity, and thus the signal, is at the maximum when the flow tube crosses its rest point (zero crossing). The Coriolis force induced twist causes a phase shift in the velocity signal that is detected by measuring the difference in the zero crossing times between the two velocity sensors. In practice this places a large accuracy burden on the time measurement circuitry. This may limit the ultimate sensitivity of mass flow measurement by this technique.
Further, the flow rate capabilities of known devices based on Coriolis technology often are limited to flow rates that are higher than desired for many applications. Moreover, existing Coriolis mass flow measuring devices only provide for mass flow sensing with no integral flow control capabilities. It has been left to the user to provide any means for controlling flow.
The present invention addresses shortcomings associated with the prior art.
In one aspect of the present invention, a capacitive pick off sensor for a mass flow measurement device is disclosed. The mass flow measurement device includes a flow sensor tube and a drive device for vibrating the flow sensor tube. The capacitive pick off sensor includes at least one conductive plate connectable to a first voltage potential and adapted to be situated adjacent the flow sensor tube which is connected to a second voltage potential. The conductive plate is positioned relative to the flow sensor tube so as to define a gap therebetween The capacitance between the conductive plate and the flow sensor tube varies due to the relative motion of the conductive plate and the flow sensor tube when the flow sensor tube is vibrated.
In another aspect of the present invention, a Coriolis mass flow sensor includes a flow sensor tube, a housing having the flow sensor tube situated therein, a drive device positioned outside the housing for vibrating the flow sensor tube, and at least one pick off sensor situated relative to the flow sensor tube so as to measure the twist in the flow sensor tube due to Coriolis force. In exemplary embodiments, the pick off sensors are situated in the housing. In further embodiments, a magnet is coupled to the flow sensor tube, and the drive device comprises an electromagnetic coil. The magnet may be a non-rare earth magnet, and more specifically, a nickel plated samarium cobalt magnet. The electromagnetic coil may comprise a power inductor.
In accordance with still further aspects of the present invention, a mass flow measurement device includes an enclosure having first and second ends, a flow body, and a first sealing member situated relative to the enclosure first end and the flow body such that the flow body and the first end are connected in a sealed manner. A user interface assembly and a second sealing member are situated relative to the enclosure second end such that the user interface assembly and the second end are connected in a sealed manner. The sealing members allow interchanging among multiple types of user interface assemblies, including a user interface assembly that is IP-65/NEMA 4X compliant, and provide additional fluid containment.